1. Field of the Invention
The present invention relates to automatic means for measuring the vertical elevation of a mass of material with respect to a given reference point and for automatically controlling the supply or depletion of material within a given area to maintain a predetermined elevation in the mass.
In its presently contemplated preferred application, the invention relates to means for automatically measuring the level of grain, aggregate, fluids or other materials which are maintained within storage containers. The field of the invention also includes automatic control systems in whic material is added to or withdrawn from a container to maintain a predetermined level of material within the container.
2. Description of the Prior Art
A commonly employed technique for measuring material levels in bins includes the use of mechanically vibrated probes which extend into the bin. In such devices, the period of vibration of the probe is transduced into an electrical signal which in turn in converted into an output representation through a suitable analog transducer. Another prior art device includes a tap like probe which extends into a container holding fluid or other material. Elastic disturbances are induced in the probe and the delay time or amplitude of the reflected mechanical movement through the probe is transduced into an electrical signal to operate an analog output device which gives an indication of material depth.
Still another prior art device includes plural electrical lines which extend through the matter to be measured. The two lines function as an electrical delay line or transmission line through which electrical signals are introduced from a suitable signal source. The reflected signals in the line are timed or measured to produce an indication of the level of material within the container.
Other prior art designs include one or more probes which extend through the matter being measured and function as variable capacitors having a capacitive reactance which varies as a function of the level of the matter present in the area of the probes.
Another of the prior art systems known to the applicant herein employs a single or plural element, high frequency resonant probe which functions as an open ended resonant coaxial cavity. When the material level is sufficiently close to the probe, an oscillator circuit is loaded to terminate resonance and signal oscillation which then initiates operation of a control system.
In many of the prior art devices described herein, the probes are relatively delicate and are subject to breakage and wear. Such probes are generally not suited for use in material such as grain, aggregate and other relatively heavy abrasive solids.
Those prior art systems which employ mechanical oscillation or other physically movable probes require complex motion inducing means as well as elaborate electrical transducing and analyzing circuitry. Moreover, such systems are often unreliable and are difficult to install an maintain.
Various of the prior art systems described herein actually function exclusively as level monitors which are sensitive to only a few predetermined levels of material in a container. Such systems do not provide output represenations or displays which show a continnuous variation of level within the container.
Prior art probes which require more than a single element are also undesirable to the extent that the container is cluttered with additional equipment. Capacitive probes are often objectionable in that extraneous conditions not related to the level of the matter being measured may cause false readings. In addition, a large noise-to-signal ratio is often present in systems where capacitance is the variable electrical characteristic being monitored to provide a depth reading.